This invention relates to particle accelerators that are usable for multiple purposes, to the simultaneous use of getter strips for two important functions in electron storage rings, to much enhanced flexibility of magnet arrangements in synchrotrons and storage rings, and to several unique and novel accelerator concepts that relate to the use of negative ions. The latter include stretcher rings, accumulator rings for ions, accelerator rings of unusually low residual activity, novel methods of accelerator alignment and tuneup, and a novel method of precision local vacuum measurement in rings under high vacuum. A stretcher ring is well known to be a storage ring in which a circulating pulse of particles that is bunched in space is caused to be stretched in time by repeated extraction of small portions of the circulating pulse. An accumulator ring is well known to be a storage ring in which a number of pulses has been loaded into the ring to produce a number of circulating particles that can be extracted to produce a pulse having more particles than the individual loading pulses.
The use of accelerators for multiple purposes. relates, in particular, to a particle accelerator system that can be used to store electrons to produce X-rays by synchrotron radiation, and also accelerate protons or other ions to be used for tissue therapy or radiography. An important use of the X-rays that are produced by the circulating electron beam will be in the lithography of integrated circuits.
The methods presently used for making integrated circuits involve the preparation of masks that are used with visible or ultraviolet light to create patterns on semiconductor materials. These patterns are etched or otherwise treated to produce desired patterns on the semiconductor material. Line spacings of the order of one micron are being produced by this method. However, such spacings are approaching the limit of resolution of visible and ultraviolet light. This resolution can be increased by the use of X-rays because of their shorter wave length. Conventional methods of generating X-rays such as the Coolidge tube produce X-rays by the impact of an electron beam on a metal target. Such devices do not produce X-rays of sufficient intensity for use in producing integrated circuits, and they produce X-rays in beams that are not sufficiently precisely defined for the purpose of making semiconductors.
One technique that has been used to expose semiconductors to X-radiation is to generate synchrotron radiation by accelerating electrons in a particle accelerator. A summary of the status of research approaches to the use of synchrotron radiation for this and other purposes is given in a publication entitled "Construction and Commissioning of Dedicated Synchrotron Radiation Facilities: Proceedings of Workshop, Oct., 1985". This is Publication BNL 51959 of the Brookhaven National Laboratory. This publication describes the status of several such facilities, including the National Synchrotron Light Source which is located at Brookhaven National Laboratory. The National Synchrotron Light Source, the Synchrotron Radiation Center at the University of Wisconsin and the Stanford Linear Accelerator Center (SLAC) Synchrotron Radiation Center in Palo Alto represent large-scale laboratories devoted to producing synchrotron radiation for research. Comparable facilities are reported in the publication described above in Japan, Germany, Great Britain, Italy, France, The People's Republic of China, Denmark and Brazil. Two third-generation synchrotron sources, featuring much higher X-ray intensities that can be achieved by insertion devices, are either under construction or in the final stages of design in the U.S. These are the Advanced Light Source at Lawrence Berkeley Laboratory and the 7-GeV Advanced Photon Source at Argonne National Laboratory. These and each of the facilities described in the Proceedings of the Workshop represents an electron storage ring with an injector that is used only occasionally to fill the ring. Each represents a research installation of major scope, served by numbers of buildings and staffs of substantial size. Such facilities are costly to build and costly to operate. They are permanently emplaced so that users must come to them to obtain the synchrotron radiation for any purpose.
In contrast to the electron storage rings described above, proton synchrotrons and cyclotrons have been and are being used in medical applications. An example of such an accelerator concept for proton radiography is shown in my U.S. Pat. No. 3,986,026, entitled "Apparatus for Proton Radiography," which is incorporated here by reference as if set forth fully. Proton therapy has been carried out for more than ten years at the Harvard Cyclotron Laboratory by physicians from the Massachusetts General Hospital utilizing the Harvard 160 MeV cyclotron. Proton therapy is carried out at facilities designed for physics use in many other parts of the world including Japan, Sweden, and the USSR, while the Lawrence Berkeley Laboratory has utilized helium ions and heavier ions for this purpose.
Proton radiography and proton therapy are normally carried out using protons in an energy range of the order of 70-250 MeV (250 MeV per nucleon, or 1000 MeV for helium ions). This energy range is sufficiently low in comparison to the design energy ranges of most present research proton accelerators that most of the experimental work done to date using protons for radiography or therapy has been performed with beams diverted from accelerators that serve large proton physics facilities. It is therefore necessary to bring the patient to the accelerator as it is difficult or impossible to locate the research accelerator at a site that is convenient to a hospital or medical facility. Similarly, the large research electron synchrotrons that are used to produce X-rays by synchrotron radiation cannot be taken to an industrial location, but instead require that the work be brought to them. It is important to design accelerators that are simple, reliable, inexpensive, and able to be utilized near a hospital or industrial location.
Several small electron storage rings have been designed specifically for production of X-rays for lithography of computer chips. These include designs from Cosy Microtech in Berlin, a compact superconducting ring; Oxford Instruments in England, a compact superconducting ring; a room-temperature ring at Brookhaven National Laboratory; a storage ring proposed by Maxwell Laboratories/Brobeck Associates in California; and several such designs in Japan. All use curved magnets, and none are suitable for alternative uses with ions or for dual uses of the injector. Such dual use has not been proposed. All of the units described above use injectors only sporadically to refill the storage ring.
A proton synchrotron of 250 MeV dedicated to proton therapy has been designed by Fermi National Laboratory in Batavia, Ill., for use at Loma Linda University Medical Center in Loma Linda, Calif., and is presently under construction. This machine is not suited for negative hydrogen ions (by reason of vacuum and peak magnetic field), has curved magnets, and with electrons would not give adequately short wavelength of synchrotron light to be useful for the lithography application. No dual use of this machine has been proposed.
In many of the electron storage rings that have been built, a problem that has placed a severe limit on the intensity of the beam achieved and the ability to maintain that beam over long periods of time, at least initially, is the trapping of positive ions in the beam. The negative charge of the electron beam represents a potential well for positive ions. These ions are produced by the ionization of the residual gas by the electron beam. They overfocus the electrons and limit the current. The problem is thus sensitive to the achievable vacuum and is exacerbated by desorption of gas from the walls of the vacuum chamber by irradiation with the X-rays produced by the electrons. A long period of time, measured in tens of Ampere-hours of beam divided by the peak achievable beam current, has often been required to reduce the desorption to the point that high currents can be accumulated or maintained. For some installations this has taken as long as a year. This problem has presented such a limit on some operations that designers of systems for producing X-rays through synchrotron radiation have turned to the acceleration of positrons. This eliminates the trapping of positive ions in the beam. However, the use of positrons as particles to be accelerated introduces considerable complexity into the operation of such an accelerator and limits its use to the type of large university facility or national laboratory described above.
The feasibility of using clearing fields to solve the ion-electron problem was demonstrated at the Aladdin facility in Wisconsin. This permitted that facility to overcome a severe current limit of a few milliamperes of stored beam. They also reported the success of single asymmetrical electrodes for this purpose, and the effectiveness of an RF voltage applied between the electrode and the accelerator wall.
The use of non-evaporable getter strips composed of an alloy of zirconium and aluminum was proposed in my U.S. Pat. No. 3,986,026 entitled "Apparatus for Proton Radiography," previously incorporated by reference. Its use was based on very successful experiments in both Europe and the United States, although such strips have not been used in any synchrotron or storage ring to date. The very high distributed pumping speed, 1 liter/sec per square centimeter of pump surface area, can lead to very high vacuum, and the use of such strips has been proposed for the 7 GeV Advanced Photon Source at Argonne and the Large Electron-Positron (LEP) collider in CERN, Switzerland. There has not been any suggestion of the use of such strips for both high-speed vacuum pumping and to provide clearing fields to eliminate the ion-electron problem.
The acceleration of negative hydrogen ions to 250 MeV in a small, weak-field synchrotron and using charge-exchange stripping extraction to utilize the protons for cancer therapy was first proposed by me at a meeting of the Proton Therapy Cooperative Group (PTCOG) at Fermilab in 1985. Slow extraction was required in order to carry out raster scanning of a tumor site and obtain nearly 100% utilization of the extracted beam. This permits keeping the required beam current to 3 nA, with the synchrotron operating at 1 Hz, including 0.4 sec. each pulse for beam extraction. This current is adequate for treatment of the majority of the cancer sites to which proton therapy is applicable in a period of 2 minutes per fraction. A fraction is that portion of a total radiation dose that is administered in a single sitting. From the point of view of economics, it is very important to keep the circulating current as low as tolerable. However, for the larger tumor sizes, radiotherapists would prefer beams of 20-30 nA. The accelerator designer could significantly increase the injection energy or the magnet aperture to meet this new requirement. Both of these alternatives would significantly increase the cost to the point that the facility would be no longer economically feasible. A more economic alternative is the use of a beam stretcher ring. The synchrotron is cycled at 10 Hz, beam is transferred as negative hydrogen ions to the stretcher ring at the peak of the synchrotron cycle, and beam is extracted for therapy from the stretcher ring for 90 of the 100 milliseconds between injection pulses.
The concept of stretcher rings for quite different purposes is not new, although all proposed have included higher field superconducting magnets. A superconducting storage ring in the ZGS tunnel at Argonne National Laboratory was proposed to save electrical power and to increase the duty cycle of experimental use of the proton beam from the existing 25% to nearly 100%. The "Energy Saver/Doubler" at Fermi National Accelerator Laboratory was built to double the energy of the existing ring by having a higher field superconducting ring at 5T in the same tunnel, and to save operating electrical energy by having very long (60-second) pulses at full field. The latter requires very little energy for the superconductors, and cycles the synchrotron much less frequently than previously. This is quite the opposite purpose proposed here. The facility is presently called the TEVATRON.
At one of the later meetings of PTCOG, a desire was expressed to carry out the fractionated cancer treatment with protons in the time that the therapist could ask a patient to hold his breath (10-15 seconds), rather than the two minutes previously discussed. The rationale for this request was that to make use of the higher precision of dose localization available with protons, patient motion could not longer be tolerated. To accomplish this order of magnitude increase in instantaneous rate with a synchrotron has seemed out of question in terms of economic feasibility. However, a combination of two accepted techniques appear to solve even this problem. These are the acceleration of negative hydrogen or deuterium ions in a rapid-cycling synchrotron, fast extraction from the synchrotron, and charge-exchange (stripping) injection into the accumulator ring. By this technique, a large number of synchrotron pulses (200-400) can be injected into the accumulator ring until enough protons for the complete irradiation are accumulated. These protons can then be extracted in less than 10 seconds in any of the standard ways. The high injection energy (up to 250 Mev) insures that the space-charge limit of the accumulator ring will be high enough to contain an adequate number of protons.
Charge-exchange injection by stripping H.sup.- ions in a thin foil to reach high intensity in a small ring is presently a very commonly used technique. It was first carried out in an operational way on the ZGS at Argonne National Laboratory, the Rapid Cycling Synchrotron of the Argonne Intense Pulsed Neutron Source, the Booster Synchrotron at Fermilab, the Alternating Gradient Synchrotron (AGS) at Brookhaven, the KEK (Japan) Booster Synchrotron, the Los Alamos Proton Storage Ring, and the Rutherford (England) Intense Neutron Source. All of these facilities are physics facilities having expensive injectors, not economically suited for the present application. In all of these facilities, the H.sup.- ion beam is produced from a linear accelerator (linac); no transfer from one synchrotron to another has been proposed before.
Most dipole magnets used in synchrotrons and storage rings are curved magnets. The only exception to this practice seems to be those rings whose circumference is so large that the sagitta of long straight magnets is small enough that it does not present a problem. The sagitta is defined as the distance along a radius of a circle from the perpendicular intersection of the radius with a chord to the circumference of the circle. Examples are the rings at Fermilab (1000 magnets in a circumference of about 3 miles) and equivalent rings in Switzerland and the USSR. Two of the advantages of short, straight magnets, that of simplicity of construction and cost were recognized in my U.S. Pat. No. 3,986,026, cited above. A very significant advantage of that choice, however, that of the flexibility of arrangement of the dipoles and quadruples for many different applications, including lithography, was not obvious or recognized at the time. This flexibility has recently allowed me to propose a facility for synchrotron radiation in which the same dipole magnets were arranged into a full energy storage ring, and a smaller number into a smaller and lower energy injector synchrotron.
Negative ions are presently in common use for rather specific applications. Sources of negative ions are required as inputs to tandem accelerators, with polarity changing from negative to positive in the high-voltage terminal to achieve additional acceleration from that terminal back to ground potential. Negative hydrogen ions are accelerated in the sector focussed cyclotron, TRIUMPF, in Vancouver, British Columbia, where they are passed through a movable foil at high energy to provide protons of variable energy outside the accelerator. No means of direct extraction of the negative hydrogen ions is provided. Intense negative ion sources at energies of 100-200 keV, with neutralization to produce neutral beams for injection into Tokamaks are in use or under development. The widespread use of H.sup.- ions for change exchange injection into synchrotrons or storage rings as a superior injection process has been discussed above. My U.S. Pat. No. 3,986,026 suggested acceleration of negative hydrogen ions to the 100-200 MeV energy range with change-exchange extraction to utilize protons in the radiographic work station. There has not to date been any suggestion of utilizing high energy (100-200 MeV) negative hydrogen ions, or any other negative ions, directly in work stations. The recent emphasis of Neutral Particle Beams (NPB) with these high energies for the Strategic Defense Initiative, in which negative ions are accelerated and then must be neutralized, has introduced a need for such ions (including H.sup.-, D.sup.-, and other negative ions) for research. Other applications not presently obvious may also emerge. Production of negative ions of these energies at relatively low intensity (and low cost) requires the concept proposed here: very high vacuum and relatively low magnetic fields to avoid removing the loosely bound electron during the acceleration process, flexible arrangement of bending and focusing magnets to accommodate a variety of possible application requirements, and low residual activity synchrotrons, especially with negative deuterium ions, for easy personnel access.
Also not recognized in the earlier concepts were the advantages of the short magnets when negative ions are circulated in the ring. These include rings with low residual activity, and the possibility of utilizing stripping extraction in many places for precision alignment and tuneup. The negative ions can also be used for local gas measurements.